Lithographic base and a method for making a lithographic printing plate therewith

ABSTRACT

The present invention provides a lithographic base comprising on a hydrophobic support a subbing layer and a hydrophilic layer containing a hydrophilic (co)polymer or (co)polymer mixture and being hardened with a hydrolyzed tetraalkyl orthosilicate crosslinking agent characterized in that said subbing layer contains a hydrophilic binder and silica, the surface area of the silica being at least 300 m2 per gram.

DESCRIPTION

1. Field of the Invention

The present invention relates to a lithographic base. More particularlythe present invention relates to an improved adhesion of the hydrophiliclithographic surface layer to the support of the lithographic base.

2. Background of the Invention

Lithography is the process of printing from specially prepared surfaces,some areas of which are capable of accepting lithographic ink, whereasother areas, when moistened with water, will not accept the ink. Theareas which accept ink form the printing image areas, generallyhydrophobic areas, and the ink-rejecting areas form the backgroundareas, generally hydrophilic areas.

In the art of photolithography, a photographic material is madeimagewise receptive to oily or greasy inks in the photo-exposed(negative-working) or in the non-exposed areas (positive-working) on ahydrophilic background.

In the production of common lithographic printing plates, also calledsurface litho plates or planographic printing plates, a lithographicbase that has affinity to water or obtains such affinity by chemicaltreatment is coated with a thin layer of a photosensitive composition.Compositions for that purpose include light-sensitive materials such aslight-sensitive polymers, diazonium salts or resins, a photoconductivelayer, a silver halide emulsion etc. These materials are then image-wiseexposed to actinic radiation and processed in the appropriate manner soas to obtain a lithographic printing plate.

In another embodiment, a silver precipitating (nucleating) agent islocated in or on top of the hydrophilic surface. An image is obtained onthe precipitating layer according to the silver salt diffusion transferprocess by contacting said precipitating layer with an exposed silverhalide emulsion in the presence of a silver halide developing agent anda silver halide solvent.

Several types of supports can be used for the manufacturing of alithographic imaging printing plate. Common supports are for exampleorganic resin supports, e.g.polyesters,and paper bases, e.g. polyolefincoated paper. These supports, if not sufficient hydrophilic bythemselves, are first coated with a hydrophilic layer forming thehydrophilic lithographic background of the printing plate.

It is known to use as hydrophilic layer in these systems a layercontaining polyvinyl alcohol and hydrolyzed tetra(m) ethyl orthosilicateand preferably also silicium dioxide and/or titanium dioxide asdescribed in e.g. GB-P-1419512, FR-P-2300354, U.S. Pat. Nos. 3,971,660and 4,284,705, EP-A-405016 and 450199 and U.S. Ser. No. 07/881,718.

In the use of such lithographic printing plates, an oleophilic (inkreceptive) image is present on a hydrophilic background. In printing,the printing plate is continuously wetted with water and ink. The wateris selectively taken up by the hydrophilic areas, the ink by theoleophilic areas of the printing surface. During the printing process,there occurs abrasion of the hydrophilic layer due to poor adhesion ofthis layer to the support either point by point or over the wholesurface. The adhesion of the hydrophilic layer to the support isadversely influenced by a higher water adsorption and thus a less rigidhydrophilic layer. Due to said abrasion the hydrophobic support comes tothe surface. This leads to ink acceptance in the non-printing areas,causing staining of the plate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lithographic basehaving on a support a hydrophilic layer of which the adhesion to thesupport is improved.

Further objects of the present invention will become clear from thedescription hereinafter.

According to the present invention there is provided a lithographic basecomprising on a hydrophobic support a subbing layer contiguous to ahydrophilic layer containing a hydrophilic (co)polymer or (co)polymermixture and being hardened with a hydrolysed tetraalkyl orthosilicatecrosslinking agent characterized in that said subbing layer contains ahydrophilic binder and silica, the surface area of the silica being atleast 300 m² per gram.

According to the present invention there is provided a method forobtaining a lithographic printing plate comprising the step(s) of makingthe above mentioned lithographic base imagewise receptive to oily orgreasy inks.

Detailed Description of the Present Invention

As hydrophilic binder in the subbing layer for the lithographic base inconnection with the present invention usually a protein, preferablygelatin may be used. Gelatin can, however, be replaced in part orintegrally by synthetic, semi-synthetic, or natural polymers. Syntheticsubstitutes for gelatin are e.g. polyvinyl alcohol, poly-N-vinylpyrrolidone, polyvinyl imidazole, polyvinyl pyrazole, polyacrylamide,polyacrylic acid, and derivatives thereof, in particular copolymersthereof. Natural substitutes for gelatin are e.g. other proteins such aszein, albumin and casein, cellulose, saccharides, starch, and alginates.In general, the semi-synthetic substitutes for gelatin are modifiednatural products e.g. gelatin derivatives obtained by conversion ofgelatin with alkylating or acylating agents or by grafting ofpolymerizable monomers on gelatin, and cellulose derivatives such ashydroxyalkyl cellulose, carboxymethyl cellulose, phthaloyl cellulose,and cellulose sulphates.

A preferred silica in the subbing layer for the lithographic base inconnection with the present invention is a siliciumdioxide of theanionic type. The colloidal silica has a surface area of at least 300 m²per gram, more preferably a surface area of 500 m² per gram.

The surface area of the colloidal silica is determined according to theBET-value method described by S. Brunauer, P. H. Emmett and E. Teller,J.Amer. Chem. Soc. 60, 309-312 (1938).

The silica dispersion may also contain other substances,e.g. aluminiumsalts, stabilising agents,biocides etc.

Such types of silica are sold under the name KIESELSOL 300 and KIESELSOL500 (KIESELSOL is a registered trade name of Farbenfabriken Bayer AG,Leverkusen, West-Germany whereby the number indicates the surface areain m² per gram).

The weight ratio of the hydrophilic binder to silica is preferably lessthan 1. The lower limit is not very important but is preferably at least0.2. The weight ratio of the hydrophilic binder to silica is morepreferably between 0.25 and 0.5.

The solids content of the subbing layer for the lithographic base inconnection with the present invention is preferably between 200 mg perm² and 750 mg per m², more preferably between 250 mg per m² and 500 mgper m².

The coating of the above defined subbing layer composition may proceedfrom an aqueous colloidal dispersion optionally in the presence of asurface-active agent.

As hydrophilic (co)polymers in the hydrophilic layer of the lithographicbase in connection with the present invention one may use, for example,homopolymers and copolymers of vinyl alcohol, acrylamide, methylolacrylamide, methylol methacrylamide, acrylic acid, methacrylic acid,hydroxyethyl acrylate, hydroxyethyl methacrylate or maleicanhydride/vinylmethylether copolymers. The hydrophilicity of the(co)polymer or (co)polymer mixture used is the same as or higher thanthe hydrophilicity of polyvinyl acetate hydrolyzed to at least an extentof 60 percent by weight, preferably 80 percent by weight. Mostpreferably polyvinyl alcohol is used in the hydrophilic layer inconnection with the present invention.

Examples of tetraalkyl orthosilicate crosslinking agents are hydrolyzedtetraethyl orthosilicate and hydrolyzed tetramethyl orthosilicate. Theamount of tetraalkyl orthosilicate crosslinking agent is at least 0.2parts by weight per part by weight of hydrophilic (co)polymer,preferably between 0.5 and 5 parts by weight, preferably 1.5 parts byweight.

The hydrophilic layer of the lithographic base preferably also containssubstances that increase the mechanical strength and the porosity of thelayer. For this purpose colloidal silica may be used. The colloidalsilica employed may be in the form of any commercially availablewater-dispersion of colloidal silica for example having an averageparticle size up to 40 nm, e.g. 20 nm. In addition inert particles oflarger size than the colloidal silica can be added e.g. silica preparedaccording to Stober as described in J. Colloid and Interface Sci., Vol.26, 1968, pages 62 to 69 or alumina particles or particles having anaverage diameter of at least 100 nm which are particles of titaniumdioxide or other heavy metal oxides. By incorporating these particlesthe surface of the layer is given a uniform rough texture consisting ofmicroscopic hills and valleys, which serve as storage places for waterin background areas.

More details about suitable hydrophilic layers for use in connectionwith the present invention can be found in e.g. GB-P-1419512,FR-P-2300354, U.S. Pat. No. 3,971,660, U.S. Pat. No. 4,284,705,EP-A-405016, EP-A-450199 and U.S. Ser. No. 07/881,718.

Various supports may be used in the lithographic base in connection withthe present invention. Examples of such supports are organic resinsupports e.g. polyethylene terephthalate film, cellulose acetate film,and polyolefin (e.g. polyethylene) coated paper. The support may becoated with a first subbing layer (a primer) containing e.g. a latex ofcopoly(vinylidenechloride/methyl methacrylate/itaconic acid) and silicato improve the adhesion of the subbing layer contiguous to thehydrophilic layer to the support.

In a first method to obtain a lithographic plate the hydrophiliclithographic base in accordance with the present invention may be coatedwith a thin layer of a heat- or photosensitive composition. The heat- orphotosensitive composition can also be present partially or completelyin the hydrophilic layer used in accordance with the present invention.Compositions for that purpose include heat- or light-sensitivesubstances such as heat-or light-sensitive polymers, diazonium salts orresins, quinonediazides, photoconductive layers, silver halide emulsionsetc. These materials are then imagewise exposed to actinic radiation andprocessed in the appropriate manner so as to obtain a lithographicprinting plate.

According to one embodiment of the present invention an imaging elementis prepared by applying a layer comprising a photopolymerizablecomposition and a silver halide emulsion layer to the lithographic baseof the present invention. After imagewise exposure of the silver halideemulsion layer and subsequent development a silver image is obtained.The thus obtained silver image is subsequently employed as a mask forthe photopolymerizable composition during an overall exposure of theimaging element. Finally the silver image and the non-exposedphotopolymerizable composition are removed so that a lithographicprinting plate is obtained.

According to another embodiment of the present invention an imagingelement is prepared by applying a layer comprising anaphthoquinonediazide compound and an alkali soluble resin to thelithographic base of the present invention. After imagewise exposure ofthe photosensitive element and subsequent development a lithographicprinting plate is obtained.

More details about suitable naphthoquinonediazide containinglithographic compositions for use in connection with the presentinvention can be found in e.g. EP-A-345016 and EP-A-508268.

According to still another embodiment of the present invention alithographic printing plate is produced by the following steps: (i)uniformly electrostatically charging a photoconductive layer, such as acoating of zinc oxide photoconductive pigment dispersed in thehydrophilic layer of the present invention by means of acorona-discharge, (ii) image-wise discharging said photoconductive layerby exposing it to electromagnetic radiation to which it is sensitive,(iii) applying electrostatically charged oleophilic toner particles todevelop the resulting electrostatic charge pattern and (iv) fixing thetoner to the photoconductive layer. Fixing is usually accomplished bythe use of heat which causes the toner resin powder to coalesce andadhere to the photoconductive layer.

More details about suitable electrophotographic lithographiccompositions for use in connection with the present invention can befound in e.g. U.S. Pat. No. 2,993,787.

In a preferred embodiment there is provided a heat recording materialcomprising the hydrophilic lithographic base of the present inventioncontaining in homogeneously distributed state throughout the entirelayer hydrophobic thermoplastic polymer particles having a softening ormelting temperature of more than 30° C. and that are capable ofcoagulating when brought above their softening or melting temperature,forming a hydrophobic agglomerate in the hydrophilic layer so that atthese parts the hydrophilic layer becomes sufficiently hydrophobic toaccept a greasy ink in lithographic printing wherein a dampening liquidis used.

Specific examples of hydrophobic polymer particles for use in connectionwith the present invention are e.g.polyethylene,polyvinyl chloride,polymethyl(meth) acrylate, polyethyl (meth) acrylate, polyvinylidenechloride, polyacrylonitrile, polyvinyl carbazole etc. or copolymersthereof. Most preferably used is polyethylene.

The molecular weight of the polymers may range from 5,000 to 1,000,000.

The hydrophobic particles may have a particle size from 0.01 μm to 50μm, more preferably between 0.05 μm and 10 μm and most preferablybetween 0.05 μm and 2 μm. The larger the polymer particles are the lessthe resolving power of the heat recording material will be.

The polymer particles are present as a dispersion in the aqueous coatingliquid and may be prepared by the methods disclosed in U.S. Pat. No.3,476,937. Another method especially suitable for preparing an aqueousdispersion of the thermoplastic polymer particles comprises:

dissolving the hydrophobic thermoplastic polymer in an organic waterimmiscible solvent,

dispersing the thus obtained solution in water or in an aqueous mediumand

removing the organic solvent by evaporation.

The amount of hydrophobic thermoplastic polymer particles contained inthe hydrophilic layer is preferably between 20% by weight and 65% byweight and more prerably between 25% by weight and 55% by weight andmost preferably between 30% by weight and 45% by weight. When too lowamounts are used the hydrophobicity produced at the exposed areas may betoo small and as a consequence ink acceptance will be poor in theseareas whereas too large amounts of the hydrophobic thermoplastic polymerparticles may result in ink acceptance in the non-image areas due to atoo large overall hydrophobicity of the hydrophilic layer.

The above described heat recording material can be exposed by actinicradiation while in contact with an original that contains a pattern ofareas that are capable of converting the radiation into heat at theseareas so that the hydrophobic thermoplastic polymer particles in thehydrophilic layer are softened or melted and coagulate in the exposedareas thereby increasing the hydrobicity of the hydrophilic layer atthese areas.

An especially suitable radiation is e.g. infrared or near infraredradiation. As an original there may be used e.g. an imaged silver halidephotographic material.

More details in connection with this embodiment can be found in e.g.U.S. Pat. No. 3,476,937 and U.S. Pat. No. 3,971,660.

In a further preferred embodiment there is provided a light-sensitivematerial comprising a diazonium salt or resin, contained inhomogeneously distributed state throughout or preferably coated from ahydrophilic solution over the hydrophilic lithographic base of thepresent invention.

Examples of low-molecular weight diazonium salt for use in the presentinvention include: benzidine tetrazoniumchloride, 3,3'-dimethylbenzidinetetrazoniumchloride, 3,3'-dimethoxybenzidine tetrazoniumchloride,4,4'-diaminodiphenylamine tetrazoniumchloride, 3,3'-diethylbenzidinetetrazoniumsulfate, 4-aminodiphenylamine diazoniumsulfate,4-aminodiphenylamine diazoniumchloride, 4-piperidino anilinediazoniumsulfate, 4-diethylamino aniline diazoniumsulfate and oligomericcondensation products of diazodiphenylamine and formaldehyde.

Examples of diazo resins useful in the present invention includecondensation products of an aromatic diazonium salt as thelight-sensitive substance. Such condensation products are known and aredescribed, for example, in DE-P-1214086. They are in general prepared bycondensation of a polynuclear aromatic diazonium compound, preferably ofsubstituted or unsubstituted diphenylamine-4-diazonium salts, withactive carbonyl compounds, preferably formaldehyde, in a strongly acidmedium.

Said light-sensitive layer preferably also contains a binder e.g.polyvinyl alcohol and may be applied to the lithographic base in athickness of 0.2 μm to 5 μm. Said presensitized imaging elementadvantageously contains water-soluble dyes such as rhodamines, sudanblue, methylen blue, eosin or triphenylmethane dyes such as crystalviolet, victoria pure blue, malachite green, methylviolet and fuchsin ordye pigments which are essentially water insoluble. Said dyes and/or dyepigments may be present in any layer comprised on the support of saidpresensitized imaging element but are preferably present in saidhydrophilic layer and/or light-sensitive layer.

Exposure of the presensitized imaging element advantageously proceedswith ultraviolet light optionally in combination with blue light in thewavelength range of 250 to 500 nm. Useful exposure sources are high ormedium pressure halogen mercury vapour lamps, e.g. of 1000 W. Since mostlithography is done by the offset process, the imaging element isexposed in such a way that the image obtained thereon is right reading.The exposure may be an exposure using optics or a contact exposure.

The diazo resin or diazonium salts are converted upon exposure fromwater soluble to water insoluble (due to the destruction of thediazonium groups) and additionally the photolysis products of the diazomay induce an advancement in the level of crosslinking of the polymericbinder or diazo resin, thereby selectively converting the surface, in animage pattern, from water soluble to water insoluble. The unexposedareas remain unchanged, i.e. water soluble.

When mounted on a printing press the printing plate is first washed withan aqueous fountain solution. To prevent this fountain solution frombeing contaminated by residual non-exposed diazo the unexposed diazoresin or diazonium salt should be removed from the printing plate beforemounting it on a printing press. This removal can be achieved by rinsingor washing the imaging element with water or an aqueous solution.

More details in connection with this embodiment can be found in e.g.FR-P-2300354 , U.S. Pat. No. 4,284,705 and EP-A-92203835.1.

In another method to obtain a lithographic plate the hydrophiliclithographic base in accordance with the present invention is used as animage-receiving element for a heat- or photosensitive composition.

In one embodiment the lithographic base in accordance with the presentinvention may be used as a receiving element in a thermal transferprocess where a hydrophobic substance or composition is information-wisetransferred from a donor element to said lithographic base. Such aprocess is described in e.g. U.S. Pat. Nos. 3,060,024, 3,085,488,3,649,268 and EP-A-502,562.

According to a preferred embodiment of the present invention a tonerimage may be transferred to the lithographic base of the presentinvention during an electrophotographic process as disclosed in e.g.U.S. Pat. No. 3,971,660 and EP-A-405,016.

According to the most preferred embodiment of the present invention alayer of physical development nuclei may be applied to the lithographicbase of the present invention. Suitable physical development nuclei foruse in accordance with the present invention are e.g. colloidal silver,heavy metal sulphides e.g. silver sulphide, nickel sulphide, palladiumsulphide, cobalt sulphide, zinc sulphide, silver nickel sulphide etc.The layer of physical development nuclei may contain a hydrophilicbinder but preferably does not contain a binder. The physicaldevelopment nuclei contained in the image receiving layer can also bepresent partially or completely in the hydrophilic layer used inaccordance with the present invention. A thus prepared element can beused as the image-receiving element in a DTR-process. According to thismethod an image-wise exposed photographic material comprising a silverhalide emulsion layer is contacted with said image-receiving element anddeveloped in the presence of a silver halide solvent e.g. thiosulphateor thiocyanate and one or more developing agents. Both elements aresubsequently separated and a silver image is formed in the layer ofphysical development nuclei comprised on the image-receiving element.More details about this process for obtaining a silver image in saidreceiving layer can be found in e.g. U.S. Pat. No. 4,649,096 orEP-A-397926. Said silver image is oleophilic while the background of theimage-receiving element is oleophobic so that a lithographic printingplate results. It may however be advantageous to improve theoleophilicity of the silver image by treating the silver image withso-called hydrophobizing agents. U.S. Pat. No. 3,776,728 describes theuse of heterocyclic mercapto-compounds, e.g. a2-mercapto-1,3,4-oxadiazole derivative as hydrophobizing agents, U.S.Pat. No. 4,563,410 discloses hydrophobizing liquids containing one ormore mercaptotriazole or mercaptotetrazole derivatives.

The following examples illustrate the invention without however limitingit thereto. All parts are by weight unless otherwise specified.

EXAMPLE 1 Preparation of the solutions for the subbing layers.

To a solution of 5.7 g of gelatin (viscosity: 19-21 mPas) in 940 ml ofwater was added 31.7 ml(11.4 g solid product) KIESELSOL 300 F (tradenamefor 30% aqueous dispersion of colloidal silica--surface area of 300 m²per g). Anionic wetting agents (0.6 g ) and biocides (1 g ) were added.

A second subbing solution was prepared by substituting KIESELSOL 300 Fby KIESELSOL 500 F (tradename for 30% aqueous dispersion of colloidalsilica--surface area of 500 m² per g).

Preparation of the hydrophilic layer.

To 440 g of a dispersion containing 21.5% TiO₂ (average particle size0.3 to 0.5 um) and 2.5% polyvinyl alcohol in deionized water weresubsequently added, while stirring, 250 g of a 5% polyvinyl alcoholsolution in water, 105 g of a hydrolyzed 22% tetramethyl orthosilicateemulsion in water and 22 g of a 10% solution of a wetting agent. To thismixture was then added 183g of deionized water and the pH was adjustedto pH=4.

Preparation of the lithographic base.

To a polyethylene terephthalate support,coated with a primer containing170 mg/m² of a latex of copoly(vinylidenechloride/methylmethacrylate/itaconic acid) and 40 mg/m² of silica with a surface areaof 100 m² /g were applied the above described solutions for the subbinglayers at a solids coverage of 750 mg/m². On top of the subbing layerswas coated the above mentioned hydrophilic layer to a wet coatingthickness of 50 g/m², dried at 30° C. and subsequently hardened bysubjecting it to a temperature of 60° C. for 1 week. The influence ofthe surface area of the silica used in the subbing layer on the scratchresistance and water absorption of a hydrophilic lithographic base isshown in table 1.

The scratch resistance, expressed in gram, was determined as follows:

immersing the material in demineralized water of 40° C. for 20 minutes,

blowing off the excessive amount of water present on top of theoutermost layers,

placing the test strips in an apparatus containing three small ballswith a diameter of 3.2 mm, the pressure on these balls is continuouslyincreased while the strips are transported through the apparatus

first ball from 30 to 100 gram

second ball from 100 to 400 gram

third ball from 400 to 1600 gram

recording the weight whereby the layers are permanently scratched tillthe support.

The amount of water absorption, expressed in g/m² was determined asfollows:

preserving the dry film for 15 minutes in a conditioning room at 20° C.and 30% RH,

covering the backing topcoat layer of the dry film with a waterimpermeable tape,

weighing the dry film,

immersing the material in demineralized water of 24° C. for 10 minutes,

sucking up the excessive amount of water present on top of the outermostlayers and

immediately determining the weight of the wet film and

calculating the measured weight differences between the wet and the dryfilm per square meter.

                  TABLE 1                                                         ______________________________________                                                 Surface area of                                                                             Scratch  Water                                                  silica        resistance                                                                             absorption                                    Number   (m.sup.2 /g)  (g)      (g/m.sup.2)                                   ______________________________________                                        1        300           >1600    2.38                                          2        500           >1600    2.26                                          ______________________________________                                    

We claim:
 1. A lithographic base comprising on a hydrophobic support asubbing layer contiguous to a hydrophilic layer containing a hydrophilic(co)polymer or (co)polymer mixture and being hardened with a hydrolyzedtetraalkyl orthosilicate crosslinking agent characterized in that saidsubbing layer contains a hydrophilic binder and silica, the surface areaof the silica being at least 300 m² per gram.
 2. A lithographic baseaccording to claim 1 characterized in that said hydrophilic (co)polymeror (co)polymer mixture has a hydrophilicity which is the same as orhigher than the hydrophilicity of polyvinyl acetate hydrolyzed to atleast an extent of 60 percent by weight.
 3. A lithographic baseaccording to claim 1 characterized in that said hydrophilic (co)polymeris polyvinylalcohol and the tetraalkyl orthosilicate crosslinking agentis hydrolyzed tetraethyl or tetramethyl orthosilicate.
 4. A lithographicbase according to claim 1 characterized in that the hydrophobic supportis an organic resin support or a polyolefin coated paper base.
 5. Alithographic base according to claim 1 wherein the surface area of thesilica is at least 500 m² per gram.
 6. A lithographic imaging elementcomprising on a lithographic base as defined in claim 1 or mixed withthe hydrophilic layer of said base a heat- or photosensitivecomposition.
 7. A lithographic imaging element comprising on alithographic base as defined in claim 1 or mixed with the hydrophiliclayer of said base a diazonium resin or a diazonium salt.
 8. Alithographic receiving element comprising on a lithographic base asdefined in claims 1 a layer of physical development nuclei.